1. Field of the Invention
The present invention relates to an image heating apparatus which can be suitably used as a fixing device to be equipped particularly in an image forming apparatus, such as a copying machine, a laser printer or a facsimile machine, using an electrophotographic process.
2. Description of The Related Art
Conventional image forming apparatuses using the electrophotographic process have a construction such as shown in FIG. 4, for example.
Referring to FIG. 4, an image forming apparatus has a photosensitive drum 201 provided as an image bearing member, a charging roller 202, a laser exposure device 203, a reflection mirror 204, a development sleeve 205, toner 206, a toner container 207, a transfer roller 208, paper 209 provided as a recording medium, a cleaning blade 210, a waste toner container 211, a fixing device 212, a paper cassette 213, a feed roller 214, a separating pad 215, and a high voltage power supply 216.
The photosensitive drum 201 rotates in the direction indicated by an arrow and is electrostatically charged uniformly by the charging roller 202 electrified by the high voltage power supply 216. Laser beam emitted from the laser exposure device 203 is reflected by the reflection mirror 204 to be irradiated upon the photosensitive drum, thereby forming an electrostatic latent image on the photosensitive drum 201. The toner container 207 is filled with the toner 206. With the rotation of the development sleeve 205, a suitable amount of toner 206 is charged and supplied to the surface of the photosensitive drum 201. The toner on the surface of the development sleeve 205 is attached to the electrostatic latent image on the photosensitive drum 201. The latent image is thereby developed and visualized as a toner image.
On the other hand, the feed roller 214 feeds recording materials as the recording medium one by one from the paper cassette 213 at a predetermined suitable timing. The separating pad 215 is placed in contact with the feed roller 214. The friction coefficient, the contact angle and the configuration of the surface of the separating pad 215 are adjusted so as to feed only one recording material at a time.
The visualized toner image on the photosensitive drum is transferred onto the recording material by the transfer roller 208. Transfer residual toner remained on the photosensitive drum without being transferred is collected in the waste toner container 211 by the cleaning blade 210. The photosensitive drum the surface of which has been cleaned is successively used in the next image forming processing cycle. Also, the recording material 209 on which the toner image is borne is heated and pressurized by the fixing device 212 to permanently fix the toner image thereon.
As the fixing device 212, a film heating type fixing device such as the one disclosed in Japanese Patent Application Laid-open Nos. 63-313182, 4-44057, or 4-44077 is conventionally used, which uses a heater having heating resistor elements formed by patterning on a ceramic substrate. The heater is energized to generate heat, by which an object to be heated is heated through a thin film.
FIG. 5 shows a cross section of an example of such a film fixing device.
Referring to FIG. 5, a heater 8 has heating resistor elements 8a formed on a ceramic substrate extending in an axial direction. The heater 8 has its front surface coated with a glass layer 8b, which is formed as a protective layer. A temperature detecting device 107 is mounted on the back surface of the heater 8 to detect the temperature of the same.
The heating resistor elements 8a are energized by an unillustrated power supply to generate heat. A central processing unit (CPU) controls the amount of energization power by driving a triac so that the temperature detected by a temperature detecting element 1 of the temperature detecting device 107 is kept constant. A fixation film 101 is a sleeve-shaped heat-resistant film of a three-layer structure. The innermost layer is a base layer through which mechanical characteristics such as torsional strength and smoothness of the fixation film 101 are controlled, and which is made of a resin such as polyimide, polyamide-imide, polyetheretherketone (PEEK), polyether sulfone (PES), or polyphenylene sulfide (PPS). An electroconductive primer layer formed of a material in which electroconductive particles such as carbon black particles are dispersed is formed on the base layer. This primer layer has the function as an adhesive for bonding a third layer and the base layer. The outermost layer, i.e., a top layer, is designed so as to optimize the resistance value and film thickness for the purpose of preventing occurrence of various image defects.
A heater holding member 9 supports the heater 8 and is formed of a heat-resistant resin such as PPS or a liquid crystal polymer. The heater holding member 9 also functions as a guide member for enabling the fixation film 101 to rotate smoothly. A fixation stay 106 made of a metal such as iron or aluminum has the function of suppressing deformation due to creep in the heater supporting member to increase the rigidity of the heater supporting member.
A pressure roller 104 has a core metal 104a made of aluminum, cast iron or the like, and a heat-resistant elastic member 104b made of silicon rubber or the like, the elastic member 104b covering the core metal 104a. The surface of the pressure roller 104 is coated with a fluororesin such as perfluoroalkoxy (PFA), polytetrafluoroethylene (PTFE), and fluorinated ethylene propylene (FEP) having mold releasing ability in use with toner.
The pressure roller 104 contacts the heater 8 while being pressed against the same with the fixation film 101 interposed therebetween. A fixation nip N is formed by the pressure-contact portions of the pressure roller 104 and the fixation film 101. The core metal 104a of the pressure roller 104 receives a rotating drive force, and the fixation film 101 is driven through the fixation nip to rotate. A recording material P on which toner T is borne is transported by the transfer roller and the photosensitive drum (both not shown) and is guided by a fixation inlet guide 105 to enter the fixation nip. Toner T on the recording material is pressed and heated on the recording material, and the toner resin is thereby softened and caused to adhere to the recording material to be permanently fixed.
The thus-arranged film heating type of fixing device can use a heater of a small heat capacity and is therefore capable of reducing a wait time (achieving a quick start) in comparison with conventional thermal roller type fixing devices. Since the film heating type fixing device is capable of the quick start, it eliminates the need for preheating during a non-printing period to enable the image forming apparatus to be designed to totally achieve an energy saving effect.
FIGS. 6A, 6B, 6C, and 6D show an example of the conventional fixing device to which a conventional temperature detecting device is attached. FIG. 6A is a plan view, FIG. 6B is a diagram showing the temperature detecting device in a free state, FIG. 6C is a cross-sectional view taken along the line VICxe2x80x94VIC of FIG. 6A, and FIG. 6D is a cross-sectional view taken along the line VIDxe2x80x94VID of FIG. 6C.
Referring to FIG. 6B, the conventional temperature detecting device has a heat-resistant elastic member 2 having a lower surface in which a temperature detecting element 1 is provided. The elastic member 2 is mounted on a temperature detecting element holding member 33 by being fitted to a temperature detecting element holding surface 33a. The temperature detecting element holding member 33 is attached to a positioning member 34 by plate springs 35a and 35b electrically insulated from each other and capable of also serving as leads for the temperature detecting element 1.
An elongated positioning hole 34a and a circular positioning hole 34b are formed in the positioning member 34 at front and rear positions. A harness 7 connected to the plate springs 35a and 35b is extended from the positioning member 34 to be connected to the CPU shown in FIG. 5.
A heater holding member 39 has positioning projections 39a and 39b formed integrally with its main portion. The positioning projections 39a and 39b are fitted in the positioning holes 34a and 34b of the positioning member 34. A hole 39c is formed in the heater holding member 39. The temperature detecting element 1 can be brought into contact with the ceramic substrate of the heater 8 exposed in the hole 39c. 
When the temperature detecting device is in an unrestrained state, the plate springs 35a and 35b are bent at a intermediate position so that the temperature detecting element holding portion 33 is in a downwardly-facing position, as shown in FIG. 6B. When the positioning member 34 is mounted on the heater holding member 39, the plate springs 35a and 35b are elastically deformed to press the contact surfaces of the temperature detecting element 1 and the heater 8 against each other.
Also, by fitting of the positioning holes 34a and 34b and the projections 9a and 9b, the position of the positioning member 34 in the radial direction is determined. The position of the positioning member 34 in the thrust direction is fixed and maintained by a fixing member (not shown).
As shown in FIG. 6C, the temperature detecting device is positioned relative to the heater holding member 39 and the heater 8 by the positioning member 34, and is connected to the temperature detecting element holding portion 34 by the plate springs 35a and 35b. The desired contact pressure between the temperature detecting element and the heater is maintained by the bending stress in the plate springs 35a and 35b. 
FIG. 7 is a graph schematically showing the relationship between the contact pressure and the detected temperature. The abscissa represents the contact pressure and the ordinate represents the output of the temperature detecting element. The curve in the graph was formed by plotting changes in output with respect to changes in contact pressure when the temperature of the heater was constant.
The temperature detecting device has such a characteristic that the detection result changes when the contact pressure changes, as shown in FIG. 7. In practical use of the temperature detecting device, therefore, a usable range is set as indicated by a portion of the curve having a small gradient in the graph. The gradient is not equal to or sufficiently close to zero. Under these circumstances, it is important, in designing the temperature detection system, to stabilize the contact pressure in order to achieve more accurate temperature detection, improvements in response speed and optimization of temperature control.
The conventional temperature detecting device and the heat fixing device using the temperature detecting device, however, have problems described below.
First, there is a problem of instability of each of the surface pressure of the contact surfaces and the pressure balance between the contact surfaces. If the bent shape of the plate springs varies due to variations of the plate springs due to some factors in manufacture of the plate springs, the stability of the contact surfaces tends to become lower. There is a possibility of failure to apply the desired pressure in some place where the temperature detecting element contacts the heater, even if the applied pressure is constant. This leads to a reduction in temperature detection accuracy and is regarded as an important consideration.
Second, the accuracy of positioning on the heater cannot be stabilized. Since in the conventional temperature detecting device the positioning member and the temperature detecting element holding member are provided separately from each other and connected by plate springs, the stability of the accuracy of relative positioning of the heater and the temperature detecting element tends to become lower if the sizes of the holding member and the positioning member vary due to variations of the temperature detecting device due to some factors in manufacture of the device. Since the heater itself has a temperature distribution, this tendency leads to a reduction in temperature detection accuracy and is regarded as an important consideration.
Third, there is a problem of the through hole being enlarged. Since in the conventional temperature detecting device the positioning member and the temperature detecting element holding member are provided separately from each other and connected by plate springs, there is a need to sufficiently enlarge the through hole 39c relative to the size of the temperature detecting element holding member in order to absorb variations of the temperature detecting device due to some factors in manufacture of the device. Therefore the region where the heater contacts neither the heater holding member nor the temperature detecting device tends to become larger. Heat is not sufficiently dissipated from this non-contact portion, so that the temperature of the heater becomes extraordinarily higher than the ambient temperature. Therefore the increase in size of the non-contact portion leads to nonuniformity of fixation heating and thermal stress damaging the heater, and this phenomenon is an important consideration.
Also, FIGS. 8A, 8B, 8C, and 8D show another example of the conventional temperature detecting means. FIG. 8A is a plan view, and FIG. 8B is a front view in a free state.
The temperature detecting means includes a temperature detecting element (e.g., a thermistor) 1a, leads 1b for supplying a current to the temperature detecting element 1a, jacketed leads 2, metallic terminals 2a attached in a caulking manner to one ends of the jacketed leads 2, first and second conductors 33, first welded portions 33a of the conductors 33 welded to the leads 1b from the temperature detecting element, second welded portions 33b of the conductors 33 welded to the metallic terminals 2a attached to the jacketed leads 2, a temperature detecting means main body 34a formed of a heat-resistant resin by insert molding inserting the first and second conductors 33, a temperature detecting means fixing portion 34b formed of a heat-resistant resin by insert molding inserting the first and second conductors 33, a heat-resistant elastic member 5, and a heat-resistant cladding 6 provided for the purpose of ensuring an electrical withstand voltage and protecting the temperature detecting element.
The heat-resistant elastic member 5 is placed along a lower surface of the temperature detecting means main body 34a. The temperature detecting element 1a is positioned substantially at a center of the lower surface of the heat-resistant elastic member 5. The heat-resistant cladding 6 is formed so as to cover the entire lower surface of the heat-resistant elastic member 5 along which the temperature detecting element 1a is placed. That is, the heat-resistant elastic member 5 is provided between the temperature detecting element 1a and the temperature detecting means main body 34a, and the heat-resistant cladding 6 protects the temperature detecting element 1a and the heat-resistant elastic member 5.
Each of the first and second conductors 33 is formed of an electroconductive plate spring member and is bent into an elbow-like shape between the temperature detecting means main body 34a and the temperature detecting means fixing portion 34b, as shown in FIG. 8B. In an unrestrained (free) state, therefore, the temperature detecting means has the first and second conductors 33 bent into an elbow-like shape between the temperature detecting means main body 34a and the temperature detecting means fixing portion 34b, as shown in FIG. 8B.
In the temperature detecting means, the temperature detecting element 1a and the jacketed leads 2 are electrically connected to each other by the leads 1b of the temperature detecting element 1a, the first welded portions 33a of the conductors 33, the conductors 33, the second welded portions 33b of the conductors 33, and the metallic terminals 2a attached to the jacketed leads 2.
FIGS. 8C and 8D are a front view and a side view, respectively, of the temperature detecting means attached to a heater holding member. The heater designated by the reference numeral 8 is placed along a lower surface of the heater holding member designated by the reference numeral 9. In this example, the heater 8 and the heater holding member 9 are a ceramic heater and a member for holding the heater in a film heating type of heat fixing device. A through hole 9a is formed in the heater holding member 9. Part of the back surface of the heater 8 placed along the lower surface of the heater holding member 9 is exposed in the inner surface (opposite from the heater placement side) of the heater holding member 9 through the through hole 9a. 
The temperature detecting means is placed along the inner surface of the heater holding member 9, the temperature detecting means main body 34a being positioned in correspondence with the through hole 9a of the heater holding member 9, the lower surface of the main body 34a (in which the temperature detecting element 1a is provided) facing downward Also, the first and second conductors 33 that are elbowed are warped in an extending direction against the force of their resilience. In this state, the temperature detecting means fixing portion 34b is fixed to the heater holding member 9. When the temperature detecting means is placed and fixed in this manner, the lower surface of the temperature detecting means main body 34a is maintained in contact with the back surface of the heater 8 in the through hole 9a of the heater holding member 9 by being pressed against the back surface of the heater 8 by the returning force of the resilience of the first and second conductors 33.
The jacketed leads 2 are connected to a temperature control circuit (not shown). The above-described temperature detecting means detects the temperature of the heater 8 as an amount of electricity by the temperature detecting element 1a and feeds back the amount of electricity to the temperature control circuit. The temperature control circuit controls the electric power supplied to the heater 8 according to the amount of electricity fed back as temperature detection information so that the temperature of the heater 8 is maintained at a predetermined point, thus controlling the temperature of the heater 8.
A primary object of use of the conductors 33 in the temperature detecting means is to enable a plate welding processing and an assembly process to be performed more easily. Electrical connections are made between the leads 1b and the jacketed leads 2 by using the conductors 33 as described above because operations for connecting the thin leads 1b of the temperature detecting element la and the metallic terminals 2a of the jacketed leads 2 directly by direct caulking or welding and for attaching the connected leads and terminals to the temperature detecting means main body 34a are difficult to perform in the case of mass production.
A secondary object of use of the conductors 33 is to use the conductors 33 as plate springs. That is, in the temperature detecting means in an unrestrained state, as described above, the first and second conductors 33 are bent into an elbow-like shape between the temperature detecting means main body 34a and the temperature detecting means fixing portion 34b as shown in FIG. 8B. When the thus-arranged temperature detecting means is mounted on the heater holding member 9, the conductors 33 are elastically deformed as plate springs as shown in FIG. 8C, thereby pressing the contact surfaces of the temperature detecting means 34a and the heater 8 against each other.
Also, the conventional temperature detecting element attached by welding after insert molding of the conductors by considering mass-producibility. To facilitate the welding, the conductors, the terminals and the leads are formed so as to have welded portions in correspondence with the two terminals of the temperature detecting element.
The temperature detecting means shown in FIGS. 8A to 8D, however, has problems described below.
1) The first problem is that the amount of heat dissipated from the conductors 33 is so large due to the increased area of the portions of the conductors 33 exposed outside that the temperature detection response is considerably low.
FIG. 9 is a graph showing the relationship between the temperature of the heater and the temperature detected by the conventional temperature detecting means. The abscissa represents the time, and the ordinate represents the actual temperature of the heater and the detected temperature. As shown in FIG. 9, the time delay in response of temperature detection by the conventional temperature detecting means is large and the temperature control cannot be optimized which is a problem. A high-speed-response temperature detecting means having a reduced time delay, such as shown in FIG. 9, is ideal for the detection system.
2) The second problem is that the possibility of damage to the heat fixing means is increased when a malfunction occurs in the electrical system.
If the time delay in temperature detection is large, there is a risk of the heat fixing device being damaged when an abnormal voltage is applied to the heater due to a malfunction in the electrical system, for example. That is, stopping energization of the heater by detecting its abnormal steep temperature rise may be delayed and the heat fixing device may be damaged before energization is stopped by detecting the abnormality.
Therefore, if the temperature detection response speed is low, the possibility of avoidance of a risk when abnormality occurs is reduced and the safety of the device cannot be ensured.
3) The third problem is that the surface pressure of the contact surfaces and the pressure balance between the contact surfaces are unstable.
Since part of each conductor is used as a plate spring, the stability of the contact surfaces tends to become lower if the bent shape varies depending on some factors in the manufacturing process, and there is a possibility of failure to apply the desired pressure in some place where the temperature detecting element contacts the heater, even if the applied pressure is constant. This leads to a reduction in temperature detection accuracy and nonuniformity of fixation in the heat fixing device and is regarded as an important consideration.
In view of the above-described problems, an object of the present invention is to provide an image heating apparatus in which a temperature detecting element is maintained in contact with a heater under suitable pressure.
Another object of the present invention is to provide an image heating apparatus having improved temperature detection accuracy.
Further, another object of the present invention is to provide an image heating apparatus comprising a heater, a temperature detecting element for detecting a temperature of the heater, a supporting member for supporting the temperature detecting element, and a biasing member for biasing the supporting member toward the heater, the biasing member biasing a surface of the supporting member opposite to the surface in which the temperature detecting element is provided.
These and other objects and features of the present invention will become apparent from the following detailed description of embodiments of the invention in conjunction with the accompanying drawings.